Grey iron castings have been in use for internal combustion engine components, notably engine blocks, cylinder heads and exhaust manifolds for many years. Their low cost, excellent castability and good machinability make them ideal for such applications. Where special requirements exist in mechanical properties, these criteria have been met through alloying. Most recently, there has been a great demand for alloyed grey iron having significantly enhanced thermal fatigue resistance while maintaining good machinability.
This interest in thermal fatigue resistance has come about because of engines running hotter to improve performance and to meet the more stringent vehicle exhaust standards. This is true of diesel engines, particularly cylinder heads. With the cylinder head, the most severe thermal fatigue condition occurs in the fire deck during engine heating and cooling. The flame face i.e. the internal surface of the cylinder head which defines a portion of the combustion chamber, is heated by the combustion gases to peak temperatures exceeding 900.degree. F. and sometimes approaching 1300.degree. F. Heat is then conducted to the opposite side of the fire deck nearest the engine coolant. This produces a steep thermal gradient in the fire deck which is sustained throughout engine operation. Once the engine is shut down, the flame face cools and the thermal gradient disappears. The thermal stress produced at the flame face on heating is a compression stress of high magnitude and during prolonged engine operation at high temperatures resulting in creep and stress relaxation. As a result, when the engine is shut down and the thermal gradient disappears, a tensile stress develops at the flame face. Repetition of this thermal stress cycle ultimately produces cracking.
Studies have shown that thermal fatigue resistance, and thus creep, is dependent upon a number of factors, including carbon equivalent, tensile strength, micro-structure and the influence of alloying. As regards the addition of alloys, the addition of molybdenum (Mo) is known to be the most effective contributor to enhancing thermal fatigue resistance. The same is true of vanadium (V). These two elements are further unique in that among traditional alloy elements, these alone produce a refinement in eutectic cell size when added to grey iron, and this is known to further enhance thermal fatigue resistance.
On the other hand, chrome, nickel and copper are known to have a very small multiplying factor on thermal fatigue resistance over and above their effect on tensile strength. However, a combination of molybdenum and chromium in an alloy has been found to be particularly beneficial because of the ability of chromium to resist the breakdown of cementite in a fully pearlitic matrix, thereby enhancing structural stability and preventing deterioration over long periods of operation and use. Chromium and vanadium are expensive, however, and can have an adverse effect on good machinability. Thus, there are many trade-offs in cost and in material characteristics in determining the most effective alloying additions to grey iron for a casting meeting the requirements for diesel engine cylinder head applications.
A further factor in producing the most effective and inexpensive alloyed grey iron casting for these applications is the methodology of foundry practices as it relates to critical alloying elements. In other words, production of these castings as a commercial practicality is based upon the extensive use of existing charge and return materials as well as alloying additions that typically result in alloy content (i.e. Ni, Si, Cr and possibly even carbon) unacceptable or unnecessary to this invention. This scrap iron includes all manner of alloying additions, including high nickel, chromium, silicon, (others) which may make unacceptable the use of such scrap for these particular purposes.